Modulation of Rubisco activase gene expression during heat stress in cotton (Gossypium hirsutum L.) involves post-transcriptional mechanisms

Inhibition of photosynthesis by heat stress involves deactivation of Rubisco and is exacerbated by the low thermal stability of Rubisco's chaperone, activase. Activase structure, activity and protein expression have been the focus of previous work examining the effect of heat stress on this enzyme. Relatively little is known about the response of activase gene expression to moderate heat stress or the extent to which transcriptional mechanisms control acclimation of photosynthesis to heat stress. In the current study, the effect of moderate heat stress on the diurnal expression of three activase genes in cotton (Gossypium hirsutum L.), including a newly identified member of the gene family, was examined. All three activase genes displayed a diurnal pattern of expression under optimal growth conditions, as determined by real-time RT-PCR. Although steady-state mRNA levels were reduced in the short-term by heat stress, transcript levels appeared to recover by the next diurnal cycle. Nuclear run-on assays utilizing real-time RT-PCR suggested mRNA stability, not synthesis, was responsible for the observed acclimatization. Identification of activase transcripts with alternative 3′-untranslated regions suggests a possible means for increasing transcript stability and hastening recovery of steady-state mRNA levels during heat stress. Together the results indicate activase gene expression is influenced by post-transcriptional mechanisms that may contribute to acclimation of photosynthesis during extended periods of heat stress.